1. Field of the Invention
The invention relates to methods and apparatus for making optical fiber preforms. More particularly, the invention relates to Rod-In-Tube (RIT) methods and apparatus having improved matching of preform core rods and overclad tubes.
2. Description of the Related Art
Optical fibers are thin strands of glass or plastic capable of transmitting optical signals containing relatively large amounts of information over long distances with relatively low attenuation. Optical fibers typically are made by heating and drawing a portion of an optical preform comprising a refractive core surrounded by a protective cladding made of glass or other suitable material. Conventionally, several processes exist for fabricating preforms, including a modified chemical vapor deposition (MCVD) process. See, e.g., U.S. Pat. No. 4,217,027, which is issued to MacChesney et al. on Aug. 12, 1980 and co-owned with this application. Other conventional processes include vapor axial deposition (VAD), outside vapor deposition (OVD) and plasma chemical vapor deposition (PCVD).
In the MCVD process, precursor gases such as SiCl4 and GeCl4 pass through a rotating substrate tube of silica glass. A torch heats the tube from the outside as the precursor gases pass therethrough, causing deposition of submicron-sized glass particles on the inside surface of the tube. Movement of the torch along the longitudinal axis of the tube in a plurality of passes builds up layer upon layer of glass to provide a preform tube. Once a suitable number of layers have been deposited, the preform tube is heated to cause it to collapse into a solid rod typically referred to as a preform rod, a core rod or a preform core rod. The preform core rod then is inserted into a glass overclad tube, which is collapsed onto the preform core rod using heat and a pressure gradient present about the overclad tube. Such process typically is referred to as the Rod-In-Tube (RIT) process. See, e.g., U.S. Pat. No. 4,820,322, which is co-owned with this application, and hereby is incorporated by reference herein.
The resulting preform or overclad preform has a core region with a first diameter (d) surrounded by a cladding region with a second or outer diameter (D). The ratio of the cladding region diameter (D) to the core region diameter (d), known as D/d, is useful in determining various performance parameters of optical fiber made from that preform. For example, to obtain optical fiber having desired transmission characteristics, the D/d ratio should be within an acceptable, but relatively narrow, range of values.
Because the range of acceptable values for this ratio typically is relatively narrow, variations in the particular physical dimensions of the core region and the cladding region, especially diameter and cross-sectional area (CSA), greatly affect the overall performance of optical fiber drawn from the preform. However, conventional methods for producing preform core rods (e.g., MCVD and VAD) do not always yield preform core rods with constant diameters or cross-sectional areas along the entire length of the preform core rod. Similarly, conventional methods for producing RIT overclad tubes do not always yield tubes with constant diameters or cross-sectional areas from one end to the other.
Accordingly, techniques such as passive tube matching are used to reduce the effects that variations in physical dimensions of conventionally-made preform core rods and overclad tubes have on the D/d ratio of the preform, and ultimately on the transmission and other performance characteristics of fiber drawn from the preform. Passive tube matching involves pairing up or matching preform core rods with overclad tubes that are dimensioned similarly or whose dimensional variations are similar. For example, a preform core rod whose average diameter or cross-sectional area (based on a number of measurements taken along the length of the preform core rod) is within a given percentage range lower than its normal or preferred value will be used with an overclad tube whose corresponding diameter or cross-sectional area also is within a given percentage range lower than its normal or preferred value. In this manner, in general, preform core rods that, on average, are smaller than normal will be inserted in overclad tubes that, on average, also are smaller than normal by a similar percentage. Such passive tube matching generally improves the dimensional consistency of the preform and thus tends to improve the quality and yield of the optical fiber drawn therefrom.
However, although passive tube matching offers some improvement of optical fiber quality and yield, it would be desirable to have available other methods and devices that further improve the dimensional consistency of the preform core and cladding regions with respect to one another, thus further improving the quality and yield of optical fiber drawn from the preforms.
The invention is embodied in a method and apparatus for making optical fiber preforms and for making optical fiber from the preforms. The method includes the steps of positioning an overclad tube around a preform core rod, heating the overclad tube along the length thereof in the presence of a pressure gradient to collapse onto the preform core to form the overclad optical fiber preform, and adjusting the radial size of a heated portion of the preform core rod and/or the overclad tube to actively match the radial dimensions of the preform core rod along the length thereof with corresponding portions of the overclad tube. The adjusting step varies the radial size of a portion of the preform core rod and/or the overclad tube, e.g., by applying compressive force to increase the radial dimensions by decreasing the axial dimensions and, alternatively, by also applying a drawing force to decrease the radial dimensions by increasing the axial dimensions (of the preform core rod and/or the overclad tube). The compressive and/or decompressive forces are applied, e.g., as the region of interest of the preform core rod and the overclad tube are being heated for the collapse of the overclad tube onto the preform core rod. The active matching afforded by the adjusting step reduces variations in the physical dimensions of the preform core rod and/or the overclad tube, which improves transmission and other performance characteristics of fiber drawn from the created preform, e.g., by maintaining a relatively constant D/d ratio of the preform.